Ion source and mass spectrometer utilizing same



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United Sttes Patent ION SOURCE AND MASS SPECTROMETER UTlLlZING SAME Filed Aug. '23, 1954, Sel. No. 451,331

4 Claims. (Cl. 250-'41.9)

This invention relates to mass spectrometers. In another aspect, it relates to apparatus for producing ionized particles.

Mass spectrometers have become useful instruments, both in the laboratory and in industrial applications, both to separate ions having different mass numbers or to analyze streams containing isotopes or elements of closely similar mass numbers. have a large evacuated vessel in which a high degree of vacuum is maintained.

Heretofore, ionization of the material to be analyzed has been carried out by producing a stream of electrons, collision between the electrons and the molecules of the material fed to the spectrometer producing ions which are then preferentially deflected, as by a magnetic or electric field, so that ions of a given mass number or range of mass numbers are preferentially collected by a charged electrode. The magnitude of the ion current at the charged electrode determines the amount of such component in the test material. Mass spectrometers have also been used for the separation of isotopes of the same element having different mass numbers.

Where electrons are used as the ionizing agent, they have been ordinarily provided by heating a filament to incandescence, as in an ordinary vacuum tube. Due to the large size of the evacuated vessel, the replacement of burned out filaments is a troublesome and vexatious problem. Also, where the material fed to the instrument is flammable or explosive, there is a fire or explosion hazard, particularly in a plant or refinery where flammable or explosive vapors may be present in the atmosphere in Finally, the magnitude of the electron currentdepends upon the amount of current fed through the filament, which can vary, and the emission characteristics of the filament change with ageing. Thus, the use of, incandescent filaments involves problems of instability.

In accordance with this invention, ionization of the material in the spectrometer is accomplished by collisions between alpha particles and molecules of the material fed to the spectrometer. The alpha particle source, which is of novel construction, operates at normal temperatures, is not incandescent, and provides ionization under very stable conditions. Also, the alpha particles, being much larger 'than electrons, are more efficient ionizing agents.

Accordingly, it is an object of the invention to provide a mass "spectrometer wherein the ionization of material fed thereto is affected by alpha particles.

It is a further object to provide an ion or alpha particle source of novel and improved character.

Various other objects, advantages and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a view of a mass spectrometer constructed in accordance with the invention;

, Figure 2 is a perspective view of the ionizing element or cartridge;

Such instruments, in general,

2,945,951 Patented July 19,

Figure 3 is a cross sectional view of one of the tubes of the cartridge of Figure 3 with the thickness of the layers of coating material exaggerated for purposes of clarity; and

Figure 4 is a view of a modified construction of mass spectrometer in accordance with the invention.

Referring now to Figure 1, the material to be analyzed or separated is fed through a tube 10 and a diffusion vessel 11, sometimes termed a molecular leak, to the interior of an evacuated tube 12. Thence, the gas flows through a series of tubes 13 arranged in the form of a bundle or cartridge 14 to a grid system 15. In the tube 14, the material is ionized by alpha particles in the manner hereinafter explained in detail, and the ions are accelerated and withdrawn from the bundle 14 by a field of the proper polarity existing in the bundle 14 by virtue of a voltage applied from a source 16 to two electrodes 17 and 18 at opposite ends of the bundle 14.

The ionized material thence passes through the grid structure 15 to a collecting electrode 19. The grid system 15 is composed of several sets 15a, 15b of electrodes each including a grounded electrode 20 and a high potential electrode 21 to which a radio frequency current is applied by a generator 22.

During their passage through the grid structure 15, the ions are accelerated by the radio frequency potential in an amount dependent upon their mass. Ions of a selected mass pass from one electrode set to the nextduring the period of the applied radio frequency signal and are, therefore, preferentially accelerated and passed to the collecting electrode 19; Ions of a mass other than the selected mass are not in resonance and accordingly are not preferentially accelerated to the collecting electrode 19. It will be further noted that a retarder grid 23 and an alpha particle trap electrode 24 are disposed between the grid assembly 15 and the electrode 19 to retard the ions before they reach the electrode 19 and remove any alpha particles before they impinge on this electrode.

The output appearing between the electrode 19 and ground is fed through an amplifier 25 to an oscilloscope 26 which can be calibrated to directly indicate the proportion of ions of a selected mass number or range of mass numbers in the material fed to the spectrometer.

Referring now to Figure 2, it will be seen that the bundle or cartridge 14 is formed from a series of tubes 30 of small cross section, the'm-aterial to befionized passing slowly through the tubes and the spaces between them as .it travels from the inlet end to the grid structure 15. The tubes 30 need not necessarily be circular in cross section, the purpose being to provide a slow move-' ment of the material through a multiplicity of passages ner hereinafter described.

In accordance with the invention, each tube 30, Figure 3, has a rigid base 31 of a smucturally'strong'matetrial, such as steel, Adhering to this base is an, inner coating 32 and an outer coating 33 of a radio-active material which emits alpha particles. r a

For best results, the alpha particle emitter should have a strength of at least 'millicuries per gram and a half life of greater than 14 days. A suitable material is Polonium 210 which has a specific activity greater than indicated, a half life of 138 days, and emits an alpha particle having an energy of 5.3 million electron volts. This material is particularly advantageous as the alpha particle source because it has a gamma/alpha ratio of about 10'- so that no extra shielding is required for a source of sutlicient activity to provide excellent results in the described application. Another suit- 7 3 able source material meeting the above conditions is Thorium 227.

In order to reduce the energy of the alpha particles, it is advantageous to provide a layer of absorption material, such as cadmium, overlying the radio-active plating material. To this end, I have shown an inner layer 34 of absorber material overlying the radio-active layer 32, and an outer layer 35 of absorber material overlying the radio-active layer 33. Any material is suitable for the absorber layers which has a high scattering cross section for alpha particles, and the use of such absorber material permits more energetic sources of alpha particles to be used. In typical installations of the invention, the length of the source cartridge can vary from 1 to 6 inches with a diameter of /4 to 2 inches. The tubes 30 can vary from the size of capillary tubes to inch in diameter with the minimum thickness necessary to retain rigidity of structure, for example, a wall thickness of inch. The thickness of the radio-active layers 32, 33 can vary from 0.001 to 0.01 inch, While that of the layers 34 and 35 can vary from 0.001 to 0.05 inch.

In this manner, as the material to be analyzed or separated passes through the tube bundle 14 at low velocity, the molecules thereof are ionized by alpha particles emitted from the layers 32, 33 of radio-active material. These alpha particles are advantageously slowed down or scattered by the layers 34, 35 of absorber material, thus increasing the effective ionizmg power of each alpha particle. Moreover, as the alpha particles traverse several of the tubes in the bundle 14, they pass through several layers of absorber material and are further slowed down with further increase in their specific ionizing ability. Thus, the described structure permits a very eificient utilization of the alpha particles, and provides a source of high stability with resultant elimination of burned out filaments and dangers of chance contact of flammable or explosive compounds with an incandescent filament.

In Figure 4, I have shown the ionizing source of the invention as applied to a mass spectrometer of the magnetic deflection type. In this instrument, there is a curved tube or vessel 37 in which a high degree of vacuum is maintained by a pump 38. The tube is surrounded by a strong magnet 39 producing a magnetic field therein.

The material to be analyzed or separated is fed from a chamber 40 through a conduit 41 to one end 42 of the vessel 37. Thence, the material passes through a tube bundle 14 of the type described inconnection with Figures 1 and 2, which has electrodes 17 and 18 arranged the same way and for the same purpose as described in connection with Figure 1.

The ionized molecules enter the main curved portion of the vessel 37 where they are subjected to the strong field of magnet 39 and are caused to travel in a curved path by this magnetic field. Ions having a proper mass number are directed by the field through an aperture or slit 43 whence they impinge upon a collecting electrode 44 connected to an amplifier and indicator 45. Ions of greater or less mass than the selected mass are deflected too much or too little to enter the slit 43 and do not reach the collecting electrode 44. Instead, these ions are removed by the pump 38.

As in the device of Figure 1, the ionizing device 14 provides a very steady and stable source of ionized material, and there is no danger of chance contact of an incandescent filament with a flammable or explosive mixture, since the alpha particle emitter does not have to be heated. Moreover, replacement of burned out filaments is not required.

While the invention has been described in connection with present, preferred embodiments thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention.

I claim:

1. In a mass spectrometer, in combination, an evacuated vessel, a cylindrical bundle of longitudinally extensive tubes disposed within said vessel, each tube being plated with a radio-active material which emits alpha particles, and a coating of an absorption material overlying said plating of radio-active material, said coating being formed from a material having a high scattering cross section for alpha particles, means for feeding material to be analyzed to said vessel at one end of said tube bundle, a pair of electrodes disposed, respectively, at opposite ends of said bundle, means for establishing a field within said bundle to propel ions out of the tube bundle, said tube bundle constituting an ionization zone, a collecting electrode spaced from said tube bundle, and means for establishing a field between said zone and said collecting electrode of such character that ions of a particular mass are preferentially transmitted to said collecting electrode.

2. In the mass spectrometer of claim 1 a collecting electrode spaced from said tube bundle, a plurality of sets of spaced electrodes positioned between said ionization zone and said collecting electrode, and means for applying a radio frequency potential between the electrodes of each set of such frequency that ions of a par ticular mass are transmitted from said ionization zone to said collecting electrode while ions of different masses are not so transmitted.

3. In the mass spectrometer of claim 1, means for establishing a magnetic field between said ionization zone and said collecting electrode of such strength that ions of a particular mass are deflected by said field and reach said one electrode while ions of different masses are not so transmitted.

4. A source of alpha particles including a series of elongated tubes arranged to form a bundle and a plating of radio-active material on the inside and outside of each tube, said radio-active material being an alpha particle emitter, a coating of an absorber material overlying each plating of radio-active material, said absorber material having a high scattering cross section for alpha particles.

References @ited in the file of this patent UNITED STATES PATENTS 2,373,151 Taylor Apr. 10, 1945 2,387,550 Winkler Oct. 23, 1945 2,495,274 Mayer Ian. 24, 1950 2,535,032 Bennett Dec. 26, 1950 2,723,349 Rylsky Nov. 8, 1955 FOREIGN PATENTS 598,525 France Dec. 10, 1925 398,722 Great Britain Sept. 21, 1933 OTHER REFERENCES Peycelon et al.: Abstract of application Serial No. 376,930, published May 25, 1943. 

